

JNITED STATES DEPARTMENT OF AGRICULTURE 

BULLETIN No. 649 

Contribution from the States Relations Service 
_ A. C. TRUE, Director 

may 

Those v^ton, D. C. PROFESSIONAL PAPER 

lierrins' 

halih- 

asJiPERIMENTS ON THE DIGESTIBILITY OF FISH.^ 

v^y A. D. Holmes, Specialist in Charge of Digestion Experiments, Office of Home 

Economics. 



April 13, 1918 



CONTENTS. 



Introduction 1 

Digestion experiments with men 3 

Preparation of fish 4 

Nature of the diet 5 

Boston mackerel 6 



Butterflsh 8 

Grayflsh 9 

Salmon 12 

Summary 14 



INTRODUCTION. 

While many sUidies have been made of the digestibihty of milk, 
cereals, fats, vegetables, and meats (especially beef), less information 
is a^'ailable regarding the digestibility of fish. SlowzofT ^ has reported 
experiments with six persons in which fish was substituted for meat 
in an' otherwise uniform diet to determine its effect on the metabolism 
of phosphorus, calcium, and magnesium. He found no change in 
the metabohsm of phosphorus; the absorption of calcium was dimin- 
ished 5 per cent, and the absorption of magnesium was increased 8 
per cent. Kiianitsyn ^ found that the protein supphed by salt cod 
was 90 per cent digested by human subjects when the fish was eaten 
alone and 94.4 per cent digested when eaten as a part of a mixed diet. 
Rozov,' in a study of the relative digestibility of the fat of smoked 
and fresh smelt, found that it was 98 per cent utihzed in the smoked 
and 97 per cent in the fresh. Slowzoff and Krawtschenko ^ report 

1 Prepared under the direction of C. F. Langworthy, Chief, Office of Home Economics. 

2 Verhandl. Gesell. Russ. Arzte St. Petersb., 76 (1909), p. 220. 

3 Nutritive Value of the Cod. Diss., Imp. Mil. Med. Acad. [St. Petersb.], 1887, pp. 56. [Russian.] 

* Comparative Assimilation of Fats from Fresh and Smoked Fish. Diss., Imp. Mil. Med. Aead.[ St. 
Petersb.], 1891, pp. 48. [Russian.] 

6 Verhandl. Gesell. Russ. ^Vrzte St. Petersb., 1907-8; abs. in Zentbl. Gesam. Physiol, u. Path. Stoflf- 
wechsels, n. ser., 4 (1909), No. 1, p. 40. 

Note. — This bulletin records studies of the digestibility of Boston mackerel, butterflsh, grayfish, and 
salmon, and is primarily of interest to students and investigators of food problems. 
28623°— IS— BuU. 649 



2 BULLETIN 649^ U. S. DEPARTMENT OF AGRICULTURE. 

that the absorption of mmeral salts is better on a diet containing 
fresh or salt fish and poorer on a diet containing dried fish than on a 
diet containing beef. Van Slyke and White,i using the rate of excre- 
tion of nitrogen in the urine as an index of the rate of protein diges- 
tion, found that boiled cod (fresh) was more rapidly digested llv*" 
boiled beef, boiled weakiish, boiled mussel, and boiled cod (5; 
Rosenfeld,^ in a study of the nutritive value of fish (sea pike a/ 
salmon), concludes that fish causes the excretion of a si^ 
amount of uric acid than meat and that fish is equal to beel . 
maintaining nitrogen equilibrium. In digestion experiments wtw 
beef and fi.sh, Atwater^ compared the amounts of protein, fat. an 
ash assimilated, and obtained the same coefficients of digestibilit}' - 
for both food materials. 

Studies of the digestibility of canned salmon have been reported 
by Milner/ who found in four experiments in which an average of 
401 grams of salmon was eaten daily for three days with a simple 
mixed basal ration consisting of bread, milk, butter, and sugar, that 
96 per cent of the protein and 97 per cent of the fat of the sahnon 
were retained by the body. 

A number of other investigators have studied the value of fish flesh 
for food purposes by means of artificial digestion experiments. 
Honigsberg ^ studied the relative digestibility of fish and found that 
pepsin digested whitefish protein more rapidly than raw and less 
rapidly than cooked beef. In a study of the digestibility of fish pro- 
tein by trypsin. White and Crozier ^ found that boiled codfish and 
dogfish digested more readily than boiled beef. Suhma ^ conducted 
experiments to determine whether tliere were differences in food in 
the raw state and that cooked at a high temperature which would 
affect the digestive process and concluded that gastric digestion wag 
much slower with cooked than with uncooked fish (sardines). This 
difference, he believed, was due to the enzyms present in the raw 
fish. Konig and Spittgerber,^ as a result of determinations of the 
composition, energy value, and constants of fish fat, and a study of 
the digestibility of fish flesh by means of artificial digestion exper- 
iments, concluded that fish flesh is as easily and completely digested 
as meat. 

In the earhest elaborate series of investigations of food materials 
made in this country, Atwater^ studied the composition of fish, 
and the results of tliis investigation contributed largely to the gen- 

1 Jour. Biol. Chem., 9 (1911), No. 3-4, pp. 219-229. 

2 Zentbl. Inn. Med., 27 (1906), No. 7, pp. 169-176. 

aZtschr. Biol., 24 (1888), No. 1, pp. 16-28; abs. in Jahresber. Tier Chem., 17 (1887), p. 418. 
< Connecticut Storrs Sta. Rpt. 1905, p. 142. 

5 Wiener Med. BI., 5 (1882), Nos. 19, pp. 582-585; 20, pp. 614-616. 

6 Jour. Amer. Cliem. Soc, 33 (1911), No. 12, pp. 2042-2048. 

7 Arch. Hyg., 75 (1912), No. 6-7, pp. 235-264. 

8 Landw. Jahrb., 38 (1909), Sup. 4, pp. 1-169. 

9 U. S. Comr. Fish and Fisheries Rpt. 1883, pp. 423-494. 



0. of D. 






DIGESTIBILITY OF FISH. 



eral knowledge of the food value of fish and its importance as a 
source of fat and other nutrients in the diet. The results of his 
studies ^ of 50 or more varieties showed that, on an average, fish 
contains over 18 per cent of protein, and about 4 per cent of fat. 
Generalizing from these data it has been pointed out '^ that fish 
may be divided on the basis of their fat content into three classes: 
Those with over 5 per cent fat such as shad, salmon, butterfish, and 
herring; those containing from 2 to 5 per cent fat such aswhitefish, 
halibut, and porgy ; and those containing less than 2 per cent fat such 
as bluefish, haddock, and cod. It is evident that fish, like meat, may 
contribute materially to the fat of the diet, particularly if the fatter 
varieties are eaten. On the basis of the protein they supply they 
also resemble meat, and this is true too with respect to the ways in 
which they are used in the diet. Accordingly, fish should be 
considered as a protein food and classed witli the meats. However, 
notwithstanding the fact that protein is essential in the diet and fat 
is supplied in a readily assimilated form in fish, the use of fish is 
small in comparison with the use of beef, pork, and mutton, which 
are also sources of animal protein. The demand for land animal 
''meats" is at present in excess of the supply, while the possible sup- 
ply of fish is believed to b6 much greater than the present demand. 
In view of the attempts which are being made to interest the 
pubhc in methods of reducing tlie consumption of meat without 
lessening tiie nutritive value and attractiveness of the diet, definite 
knowledge of the food value of fish is of especial importance. And 
so, as part of the studies of the food value and uses of fish in the home, 
which are being undertaken, it seemed desirable to study the digesti- 
bility of some varieties of fisli taken to be representative of general 
types, including some which are well known and one, grayfish, 
which is comparatively new in the American market. 

DIGESTION EXPERIMENTS WITH MEN. 

Seven young men (medical and dental students) who had gained 
experience in other investigations of like character, served as sub- 
jects in this investigation. They were all normal individuals of good 
health, and reasonably active. During the experimental period they 
were requested to observe their usual routine as regards amount of 
exercise taken, hom's of eating, etc. From their knowledge of physi- 
ology and previous experience in this type of work, they were suf- 
ficiently informed of the nature of their duties to appreciate the im- 
portance of carefully following the directions given them. 

For the purpose of this investigation a special fore period and after 
period were not considered necessary, and accordingly the subjects 

1 Loc. cit. and U. S. Dept. Agr., Office Expt. Stas. Bui. 28 (1906), rev. ed., pp. 45-50. 

2 U. S. Dept. Agr., Farmers' Bui. 85 (1898), p. 14. 



4 BULLETIN 649, U. S. DEPARTMENT OF AGRICULTURE. 

were allowed to follow their customary dietary routine preceding and 
following the experimental period. For the purpose of identifying 
the feces of the experimental period, three or fom' gelatin capsules 
containing about 0.3 gram of pulverized charcoal were taken with 
the first meal of the experimental period and with the first meal fol- 
lowing it ; the separation of the feces due to the diet imder investiga- 
tion was easily made at the lines of demarcation made by the por- 
tions dully colored by the charcoal. 

Inasmuch as this study is concerned principally with the coefficient 
of digestibility of the protein and fat of the fish, no attempt was made 
to maintain a nitrogen equilibrium or uniform body weight of the sub- 
jects. The urine resulting from the experimental periods was not 
collected, for it was considered that any constituents of the foods 
which had been sufficiently broken down to appear in the urine 
had undergone the process of digestion; furthermore, the results ob- 
tained by collecting and analyzing the urine of a short test period 
are not entirely conclusive since the urine can not be separated as 
satisfactorily as the feces. 

PREPARATION OF FISH. 

In this study of the digestibility of different types of fish, fresh 
butterfish and Boston mackerel were used, and canned grayfish and 
canned salmon. A fish loaf seemed to be the best form in which to 
prepare the fish for eating, since sufficient quantities for the entire 
experimental period could be prepared at one time. Furthermore, 
it was easy to prepare a fish loaf having a uniform composition and 
one which would not change materially on standing by the settling 
out of fat or evaporation of water. 

The butterfish and mackerel received a preliminary cooking before 
being incorporated in the fish loaf. The fish, after being cleaned, were 
thoroughly washed and placed as close as possible to each other in a 
covered cooker, water was added, and they were cooked for one-half 
hour. They were not boiled, but steamed in a very small quantity 
of water which prevented browning or sticking to the pan. To 
prevent any loss due to extracted fat and protein, the water in 
which the fish were steamed was retained and mixed with the fish 
meat, Be3rthien ^ having reported that the water in which fish 
were boiled contained 8.8 to 11.3 per cent of the total fish protein. 
After this preliminary cooking of the butterfish and mackerel, the 
bones, any pieces of fins, etc., were removed and the fish meat was 
cut in an ordinary household meat cutter. The bones and bits of 
skin were removed from the canned grayfish and salmon, and the 
sohd meat was minced in a meat cutter. From this point the prepa- 

1 Pharm. Centralhalle, 47 (1906), p. 140. 



DIGESTIBILITY OF FISH. 5 

ration of the fish loaf was identical for the canned and fresh fish. 
In each instance a quantity of the fish to be studied sufficient for the 
entire test period, after being mixed with salt and pepper, was very 
thoroughly and uniformly mixed and baked for two to three hours 
in a moderate oven. Wliatever crust formed during baking was 
removed, and the remaining portion was again thorouglily mixed 
in order to secure a uniform product, after which a sample was 
taken for analysis. 

NATURE OF THE DIET. 

Inasmuch as experience has shown that in studying the digestibility 
of a single food it is desirable to supply the food material under con- 
sideration as a part of a simple mixed diet, a suitable basal ration 
was served with the fish loaf. It consisted of boiled potatoes, crackers, 
apple sauce, sugar, tea or coffee, and a little lemon juice as a condi- 
ment in some cases. In accordance with the usual custom, a sufficient 
supply of the special food under consideration (fish loaf) and of the 
other foods was prepared in advance for the whole experimental 
period. The fish loaf was kept in a refrigerator at 15° C. and remained 
fresh and in good condition, as did also the potatoes, which were 
boiled, mashed, and thoroughly mixed to insure uniform composi- 
tion. The apples, which were eaten raw, were of good grade and 
pleasant flavor. The crackers or "biscuits" used were taken from 
a large lot and assumed to be of uniform composition. Though no 
attempt was made to have all eat like amounts, the subjects were 
urged to eat liberally of the fish loaf and moderately of the crackers 
and potatoes, following their individual preferences with respect to 
the apples and the tea or coffee. 

As a whole, the ration, though it contained no added fat and oidy 
moderate amounts of carbohydrates, was reasonably generous as 
regards protein and energy and was varied enough not to become 
tiresome. 

The food for each man for each meal was weighed in advance and 
kept separate. All remaining uneaten was weighed. The difference 
between the amount furnished and the amount remaining repre- 
sented the amount eaten. 

Samples of the food were reserved for analysis. The small amount 
of lemon juice (on an average, 35 grams per day), which was eaten 
with the butterfish and the Boston mackerel, was disregarded in 
computing the food value of tile diet. 

The feces were collected, sampled, and analyzed by the methods 
followed in the department's digestion experiments, of which the 
present investigation forms a part.^ 

• U. S. Dept. Agr. Bui. 310 (1915), pp. 23. See list on last page. 



6 BULLETIN 649, U. S. DEPARTMENT OF AGRICULTURE. 

BOSTON MACKEREL. 

The investigations reported in tliis paper, whicli form a part of 
an extended study of the digestibility, nutritive value, and uses in 
the home of fish and fish products, have to do particularly with the 
digestibiUty of protein. One of the varieties of fish, Boston mack- 
erel, here studied in comparison with other sorts of fish, has, however, 
been considered from the standpoint of the relative digestibiUty of 
animal fats of different kinds in an earlier bulletin.' 

Boston mackerel {Scomber scomhrus, linnseus) is a highly fla- 
vored fisli, which can be compared to moderately fat meat in food 
value, since it supplies good amounts of both protein and fat. This 
fish, which is found throughout the north Atlantic, spends the winter 
months in deep water; in the spring, schools rise to the surface and 
approach the land. This fish forms one of the chief products of 
the New England fisheries, the catch during the year 1916 amounting 
to approximately 16,000,000 pounds. The fish used in this inves- 
tigation were purchased at a local market and weighed, after clean- 
ing, approximately 3 pounds each. They were procured at the 
height of the season, were in prime condition, and when prepared 
in the form of a fish loaf made a most appetizing dish. While the 
Boston mackerel possesses a characteristic flavor which is quite 
pronounced in the boiled or fried fish, this flavor was not evident in 
the fish loaf. 

Thi-ee subjects Hving under normal conditions, who had acquired 
considerable experience in work of this kind in connection with the 
determination of the digestibility of some of the common edible fats 
of animal and vegetable origin, assisted in this study. The results 
which were obtained in the three-day test period follow. 

» U.S.Dept.Agr.Bul.507 (1917), p. 16. 



DIGESTIBILITY OF FISH. 7 

Data of digestion experiments with Boston mackerel in a simple lyiixed diet. 





Weight 
of food. 


Constituents of foods. 


Experiment, subject, and diet. 


Water. 


Protein. 


Fat. 


Carbo- 
hy- 
drates. 


Ash. 


Experiment No. 444, subject D. G. G.: 

Boston mackerel (in form of fish loaf) 


Grams. 

1,496.0 
439.0 
284.0 

1,320.0 
171.0 


Grams. 

926.0 

331.5 

19.6 

1, 116. 7 


Grams. 

323.0 

11.0 

23.0 

5.3 


Grams. 

199.9 

.4 

38.0 

6.6 


Grams. 


Grams. 
47.1 


Potato 


91.7 
201.1 
187.4 
171.0 


4.4 
2.3 
4.0 




Fruit 


Sugar 












Total food consumed 


3,710.0 
67.0 


2,393.8 


362.3 

19.3 

343.0 


244.9 

10.1 

234.8 


651.2 
31.9 
619.3 


57.8 
5.7 
52.1 


Fetes 


Amount utilized 










Per cent utilized 






94.7 


95.9 


95.1 


90.1 








Experiment No. 446, subject K. L. S.: 

Boston mackerel (in form of fish loaf) 


1, 184. 
227.0 
243. 

1,376.0 
58.0 


732.9 

171.4 

16.8 

1, 164. 1 


255.6 
5.7 
19.7 
5.5 


158.2 

.2 

32.6 

6.9 




37.3 


Potato 


47.4 
172.0 
195.4 

58.0 


2.3 
1.9 
4.1 


Crackers 


Frait 


Sugar 












Total food consumed 


3, 088. 
53.0 


2, 085. 2 


286.5 

21.9 

264.6 


197.9 

7.5 

190.4 


472.8 

16.2 

456.8 




Feces 


7.4 
38.2 


Amount utilized 








Per cent utilized 






92.4 


96.2 


96.6 


83 8 










Experiment No. 447, subject O. E. S.: 

Boston mackerel (in form offish loaf) 


1,348.0 
' 476. 

171.0 
1,594.0 

165.0 


834.4 

359.4 

11.8 

1,348.5 


291.0 
11.9 
13.8 
6.4 


180.1 

.5 

22.9 

8.0 




42.5 




99.5 
121.1 
226.3 
165. 


4.7 
1 4 


Crackers 




4.8 


Sugar 












Total food consumed 


3, 754. 
70.0 


2,554.1 


323.1 

28.6 

294.5 


211.5 
12.2 
199.3 


611.9 

21.6 

590.3 


53 4 




7.6 

45 8 


Amount utilized 












Per cent utilized 






91.1 


94.2 


96.5 


85 8 




1, 172. 4 






Average food consumed per subject per day 


781.4 


108.0 


72.7 


192.9 


17.4 



Summary of digestion experiments with Boston mackerel in a simple mixed diet. 



Experiment No. 


Subject. 


Protein. 


Fat. 


Carbo- 
hydrates. 


Ash. 


444 


D. G. G 


Per cent. 
94.7 
92.4 
91.1 


Per cent. 
95.9 
96.2 
94.2 


Per cent. 
95.1 
96.6 
96.5 


Per cent. 
90 1 


446 


R.L.S 

O.E.S 

Average 


83 8 


447 


85 8 








92.7 


95.4 


96.1 


86. 6 









An average of 108 grams of protein, 73 grams of fat, and 193 
grams of carbohydrates was consumed per man daily during the test 
periods with Boston mackerel, the fuel value of the diet being 1,861 
calories. These constituents were found to be 92.7 per cent, 95.4 
per cent, and 96.1 per cent digested, respectively. The coefficients 
of digestibility for the iish protein and fat were found to be 93.1 per 
cent and 95.2 per cent, respectively, when allowance was made for 
the undigested protein and fat resulting from the basal ration. 



BULLETIN 649, XJ. S. DEPARTMENT OF AGRICULTURE. 



BUTTERFISH. 

Butterfish (Poronotus triacanthus) was chosen for one series of 
studies as an excellent fish of the type commonly used as pan fish. 
Butterfish occurs on the Atlantic and Gulf coasts from Maine to Texas, 
and during the year 1915 the catch of the coastal fisheries of New 
York and New Jersey amounted to over 5,000,000 pounds. At times 
it is taken in such abundance that a glut in the market results, and, 
since this fish is usually sold fresh, great quantities are wasted. The 
butterfish which was studied in the experiments here reported was of 
a good commercial grade, purchased in the local market, and it was 
believed to be representative of this type of fish procured under the 
ordinary trade conditions. 

The three subjects who assisted in the study of the digestibility of 
butterfish, following the same routine which has hitherto proved en- 
tirely satisfactory, ate an average of 105 grams of protein, 37 grams 
of fat, and 208 grams of carbohydrates, daily, the fuel value of the 
diet being 1,585 calories. The data obtained for the three-day test 
period are included in the following table: 

Data of digestion experiments with butterfish in a simple mixed diet. 





Weight 
of food. 


Constituents of foods. 


Expcnment, subject, and diet. 


Water. 


Protein. 


Fat. 


Carbohy- 
drates'. 


Ash. 


Experiment No. 452, subject D. G. G.: 

Butterfish (in form of fisli loaf) . . . . 


Grams. 

1,533.0 
115.0 
322.0 

1,259.0 
189.0 


Grams. 

1, 120. 9 
86.8 
22.2 

1, 065. 1 


Grams. 

303.7 

2.9 

26.1 

5.0 


Grams. 
74.5 

0.1 
43.1 

6.3 


Grams. 


Grams. 
33.9 


Potato 


24.0 
228.0 
178.8 
189.0 


1.2 


Crackers 


2.6 


Fruit 


3.8 


Sugar 














Total food consumed 


3,418.0 
46.0 


2,295.0 


337.7 

22.8 

314.9 


124.0 

7.5 

116.5 


619.8 

n.3 

608 5 


41.5 


Feces 


4.4 


Amount utilized 




37.1 










Per cent utilized 






93.2 


94.0 


98.2 


89.4 










Experiment No. 454, subject R. L. S.: 

Butterfish (in form of fish loaf) 


1,419.0 
139.0 
355.0 

1,423.0 
163 


1,037.6 

104.9 

24.5 

1,203 8 


281.1 

3.5 

28.8 

5.7 


69.0 
0.1 

47.6 
7.1 




31.3 


Potato 


29.1 
251.3 
202.1 
163.0 


1.4 


Crackers 


2.8 


Fruit ... 


4.3 


Sugar 














Total food consumed 


3,499.0 
60.0 


2,370.8 


319.1 

24.9 

294.2 


123.8 

9.5 

114 3 


645.5 

16.5 

629.0 


39.8 


Feces 


9.1 


Amount utihzed 




30.7 










Per cent utilized 






92.2 


92.3 


97.4 


77.1 










Experiment No. 455, subject 0. E. S.: 

Butterfish (in form of fish loaf) 


1, 290. 
569.0 
122.0 

1,474.0 
196.0 


943.2 
429.6 

8.4 
1,247.0 


255.6 
14.2 
9.9 
5.9 


62.7 
0.6 

16.3 
7.4 




2S.5 


Potato 


118.9 

86.4 

209 3 

196. 


5.7 


Crackers 


1.0 


Fruit 


4.4 


Sugar 














Total food consumed 


3,651.0 
68.0 


2, €28. 2 


285.6 

29.6 

256.0 


87.0 
14.5 
72.5 


610.6 

14.3 

596.3 


39.6 


Feces . . 


9.6 


Amount utilized 




30.0 










Per cent utilized 






89.6 


83.3 


97.7 


75.8 










Average food consumed per subject per day 


1,174.2 


810.5 


104.7 


37.2 


208,4 


13.4 



DIGESTIBILITY OF FISH. 9 

Summary of digestion experiments xoith hutterjish in a simple mixed diet. 



Experiment No. 


Subject. 


Protein. 


Fat. 


Carbohy- 
drates. 


Ash. 


452 


D.G.G -. 

R.L.S 

O. E.S.. .. 


Per cent. 
93.2 
92.2 
89.6 


Per cent. 
94.0 
92.3 
83.3 


Percent. 
98.2 
97.4 
97.7 


Per cent . 
89.4 
77.1 
75.8 


454 


455 




Average 




91.7 


89.9 


97.8 


80.8 







In the digestion experiments made witli butterfish, the subjects ate 
an average of 471 grams of fish daily, which suppUed 93 grams of 
protem and 23 grams of fat. The protein, fat, and carbohydrates 
of the total diet were fomid to be 91.7 per cent, 89.9 per cent, and 

97.8 per cent digested, respectively. Tlie values 91.7 per cent and 

89.9 per cent for the digestibiUty of the protein and fat of the total 
diet become 91.9 per cent and 86.4 per cent, respectively, if allow- 
ance is made for the undigested protein and fat resulting from the 
basal ration. The estimated value, 86.4 per cent, for the digesti- 
biUty of tlie fat of butterfisli is somewhat lower than that of the 
other fish fats here reported. This lower value is no doubt in part 
due to the '4ieaping up of errors" involved in estimating the diges- 
tibiUty of a fat of a single food when it represents so small a por- 
tion of the total fat eaten. The subjects reported that they re- 
mained in normal physical condition during the experimental 
periods, except that subjects D. G. G. and O. E, S. reported that 
the diet produced a constipating effect, which was due, no doubt, 
to its very complete utiUzation. 

CRAYFISH. 

The grayfish (Squalus acanthias, Linnaeus), wliich is very abundant 
and easy to catch, though known to be wholesome, of good flavor, 
and usable for many appetizing dishes, has not been utiUzed to any 
extent in this comitry for food purposes, but has been considered 
largely as a som'ce of oil and fish scrap, a fishery industry by-product 
of value for fertilizer material. Recently the Bureau of Fisheries 
has devoted considerable attention to the possible use of this fish in 
human nutrition and is of the opinion that it constitutes a cheap and 
very wholesome food. 

In the literature consulted, no reports were foimd of the digesti- 
biUty of grayfish. In order to judge of the value of the grayfish in 
the dietary, it seemed desirable to obtain information on this point, 
and, accordingly, tests were made m which canned grayfish was 
served in the form of ^ fish loaf in conjmiction with the simple basal 
ration employed in the other tests reported in this paper. The canned 
fish used for this study was suppUed by the Bureau of Fisheries and was 
taken to be representative of a large pack put up by a commercial 
concern under the direction of the Bureau of Fisheries. 



10 



BULLETIN 649^ U. S. DEPARTMENT OF AGRICULTURE. 



Denis ^ in a study of the blood of a number of fishes found urea iu 
the blood of the grayfish. An analysis of the canned grayfish showed 
that the flesh of this fish contained both amnaoniacal and urea 
nitrogen. The amount of nitrogen present in this form is, however, 
relatively small, and when allowance was made in the experiments 
wliich follow for this nonprotein nitrogen it was found that the 
value for the coefficient of digestil)ility of protein was not materially 
changed. 

Eight digestion experiments have been made with the grayfish 
and the results obtained for the three-day test period are reported in 
the following table: 

Data of digestion experiments with grayfish in a simple mixed diet. 





Weight 
of food. 




Constituents of foods. 




Experiment, subject, and diet. 


Water. 


Protein. 


Fat. 


Carbo- 
hy- 
drates. 


Ash. 


Experiment No. 535, subject H. R. G.: 


G rams. 

1,409.0 
486.0 
400.0 
581.0 


Grams. 

971.2 

366.9 

27.6 

491.5 


Grams. 

244.3 

12.1 

32.4 

2.3 


Grams. 

160.8 

0.5 

53.6 

2.9 


Grams. 


Grams. 
32.7 


Potato 


101.6 

283.2 

82.5 


4.9 


Crackers 

Fruit 

Sugar 


3.2 
1.8 


















2, 876. 
64.0 


1,857.2 


291.1 
37.8 
253.3 


217.8 
7.3 

210.5 


467.3 

15.3 

452.0 


42.6 




3.6 






39.0 
















87.0 


96.6 


96.7 


91.5 










Experiment No. 537, subject P. K.: 


1,621.0 
507.0 
529.0 
890.0 


1,117.3 

382.8 

36.5 

752.9 


281.1 
12.7 
42.9 
3.6 


185.0 
0.5 
70.9 
4.4 




37.6 


Potato 


105. 9 
374.5 
126.4 


5. 1 




4.2 


Fruit 


2.7 


Sugar 


















Total food consumed 


3,547.0 
39.0 


2,289.5 


340.3 

17.7 

322.6 


260.8 

8.5 

252.3 


606.8 

8.1 

598.7 


49.6 




4.7 






44.9 
















94.8 


96.7 


98.7 


90.5 










Experiment No. 538, subject C. J. W.: 

Grayfish. (in form of fish loaf) 


1,331.0 

452.0 

202.0 

944.0 

12.0 


917.4 

341.3 

13.9 

798.6 


230.8 
11.3 
16.4 
3.8 


151.9 

0.4 

27.1 

4.7 




30.9 


Potato 


94.5 
143.0 
134.1 

12.0 


4.5 




1.6 


Fruit .' 


2.8 


Sugar 














Total food consumed 


2,941.0 
65.0 


2,071.2 


262.3 

27.7 
234.6 


184.1 

13.9 

170.2 


383.6 

14.9 

368.7 


39.8 




8.5 


Amount utilized 




31.3 










Per cent utilized 






89.4 


92.4 


96.1 


78.6 










Experiment No. 547, subject H. R. G.: 


1,142.0 
425.0 
343.0 
599.0 


762.2 

320.9 

23.7 

506.8 


219.7 
10.6 
27.8 
2.4 


132.2 
0.4 
4G.0 
3.0 




27.9 


Potato 


88.8 
242.8 
85.0 


4.3 


Crackers 


2.7 


Fruit 


1.8 


Sugar 


















Total food consumed 


2,509.0 
49.0 


1,613.6 


260.5 
26.7 
233.8 


181.6 

7.6 

174.0 


416.6 

9.7 

406.9 


36.7 


Feces 


5.0 


Amount utilized 




31.7 










Per cent utilized 






89.8 


95.8 


97.7 


86.4 











1 Jour. Biol. Chem., 16 (1913), No. 3, pp. 389-393. 



DIGESTIBILITY OF FISH. 11 

Data of digestion experiments with grayfish in a simple mixed diet — Continued. 





Weight 
of food. 


Constituents of foods. 


Experiment, subject, and diet 


Water. 


Protein. 


Fat. 


Carbo- 
hy- 
drates. 


Ash. 


Experiment No. 548, subject A. J. H.: 

Grayfish (in form of fish loaf) 


Orams. 
1,097.0 


Grams. 
732.1 


Orams. 
211.1 


Grams. 
127.0 


Grams. 


Grams. 
26 8 


Potato 








76.0 

168.0 

4.0 


5.2 
142.1 


6.2 
0.7 


10.2 
0.8 


53.8 

23.9 

4.0 


6 


Fruit 


5 


Sugar 














Total food consumed 


1,345.0 
19.0 


879.4 


218.0 

6.5 

211.5 


138.0 

5.7 

132.3 


81.7 

3.7 

78.0 


27 9 




3 1 






24 8 










Per cent utilized 






97.0 


95.9 


95.5 


88 9 










Experiment No. 549, subject P. K.: 


1,634.0 
500.0 
463.0 
926.0 


1, 090. 5 

377.5 

32.0 

783.4 


314.4 
12.5 
37.5 
3.7 


189.2 
0.5 
62.0 
4.6 




39.9 




104.5 
327.8 
131.5 


5 




3 7 


Fruit 


2.8 




















Total food consumed 


3, 523. 
36.0 


2, 283. 4 


368.1 

15.4 

352.7 


256.3 

9.9 

246.4 


563.8 

6.1 

557.7 


51 4 




4.6 






46 8 




1 








95.8 


96.1 


98.9 


91.1 




1 




Experiment No. 579, subject P. K.: 


1,175.0 
434.0 
416.0 

1, 03S. 
35.0 


773.7 
327.7 

28.7 
878. 1 


230.5 

10.9 

33.7 

4.2 


153.0 

0.4 

55.8 

5.2 




17.8 


Potato 


90.7 

294.5 
147.4 
35.0 


4.3 




3.3 


Fruit 


3.1 
















Total food consumed 


3,098.0 
34.0 


2,008.2 


279.3 

13.6 

265.7 


214.4 

10.0 

204.4 


567.6 

5.9 

561.7 


28.5 




• 4.5 


Amount utilized 




24.0 














95.1 


95.3 


99.0 


84.2 










Experiment No. 580, subject C. J. W.: 


1,151.0 
462.0 
185.0 

1, 168. 
112.0 


757.9 

348.8 

12.7 

988.1 


225.8 
11.5 
15.0 

4.7 


149.9 
0.5 
24.8 
5.8 




17.4 




96.6 
131.0 
165.9 
112.0 


4.6 




1.5 


Fruit 


3.5 


Sugar 
















3,078.0 
58.0 


2, 107. 5 


257.0 
22.8 
234.2 


181.0 
15.5 
165.5 


605.5 

11.5 

494.0 


27.0 


Feces 


8.2 






18.8 










Per cent utilized 






91.1 


91.4 


97.7 


69.6 










Average food consumed per subject per day 


954.9 


629.6 


94.9 


68.1 


149.7 


12.6 



Summary of digestion experiments with grayfish in a simple mixed diet. 



Experiment No. 


Subject. 


Protein. 


Fat. 


Carbo- 
hydrates. 


Ash. 


535 


H.R. G 


Per cent. 
87.0 
94.8 
89.4 
89.8 
97.0 
9.5.8 
95.1 
91.1 


Per cent. 
96.6 
96.7 
92.4 
95.8 
95.9 
96.1 
95.3 
91.4 


Percent. 
96.7 
98.7 
96.1 
97.7 
95.5 
98.9 
99.0 
97.7 


Percent. 
91.5 


537 


P. K 


90.5 


538 


C. J. W 

H. R. G 


78.6 


547 


86.4 


548 


A. J.H 


88.9 


549 ■ 


P. K 


91.1 


579 


P.K 


84.2 


680 


C.J. W 


69.6 




Average... 






92.5 


95.0 


97.5 


85.1 









12 



BULLETIN 649, U. S. DEPAETMENT OF AGEICULTUEE. 



Considering the diet as a whole, 95 grams of protein, 68 grams of 
fat, and 150 grams of carbohydrates were eaten per day, of which 
92.5 per cent of the protein, 95 per cent of the fat, and 97.5 per cent 
of the carbohydrates were digested. The fuel value of the diet was 
1,592 calories. The estimated digestibility of grayfish protein and 
fat as obtained by making allowance for the unutilized protein and 
fat resulting from the accessory foods of the diet are 92.8 per cent 
and 94.3 per cent, respectively. 

The four different subjects who assisted in the eight experiments 
conducted to determine the digestibility of grayfish ate on an average 
440 grams of grayfish daily as compared with 471 grams of butter- 
fish, 448 grams of mackerel, and 355 grams of salmon, which would 
indicate that this fish was eaten with as much relish as the other 

fish studied. 

SALMON. 

Since difficulty was experienced in securing fresh salmon for the 
purpose of this investigation, a good grade of canned salmon (Chinook 
salmon, Oncorhynchus tschawytsclia) was used instead in the experi- 
ments here reported. The canned salmon was purchased without 
regard to any commercial brand from a wholesale grocer, and was 
taken to represent this type of fish in general use. 

The results obtained in tiiree-day test periods with salmon are 
given in the table which follows: 

Data of digestion experiments ivith salmon in a simple mixed diet. 





Weight 
of food. 


Constituents of foods. 


Experiment, subject, and diet 


Water. 


Protein. 


Fat. 


Carbohy- 
drates. 


Ash. 


Experiment No. 561, subject H. R. G.: 
Salmon (in form of fish loaf).. 


Grams. 

1,004.0 
411.0 
297.0 
614.0 
23.0 


Grams. 

621.8 

310.3 

20.5 

434.8 


Grams. 

249.8 

10.3 

24.0 

2.1 


Grams. 

108.8 

0.4 

39.8 

2.6 


Grams. 


Grams. 
23.6 


Potato 


85.9 

210.3 

73.0 

23.0 


4.1 




2.4 


Fruit 


1.5 


Sugar . 












Total food consumed 


2,249.0 
46.0 


1,387.4 


286.2 

23.3 

262.9 


151.6 

4.7 
146.9 


392.2 
13.5 

378 7 


31.6 


Feces 


4.5 






27.1 










Per cent utilized 




• 


91.9 


96.9 


96.6 


85.8 










Experiment No. 562, subject A. J. H.: 


854.0 


528.9 


212.5 


92.5 




20.1 


Potato 






Crackers 


104.0 
103.0 
24.0 


7.2 
87.2 


8.4 
0.4 


14.0 
0.5 


73.6 
14.6 
24.0 


6.S. 


Fruit 


0.3 


















1, 085. 
42.0 


623.3 


221.3 

18.3 

203.0 


107.0 
12.1 
94.9 


112.2 

6.7 

105.5 


21.2 


Feces... 


4.9 






16.3 










Per cent utilized 






91.7 


88.7 


94.0 


76.9 











DIGESTIBILITY OF FISH. 13 

Data of digestion experiments loith salmon in a simple mixed diet — Continued. 





Weight 
of food. 


Constituents of foods. 


Experiment, subject, and diet. 


Water. 


Protein. 


Fat. 


Carbohy- 
drates. 


Ash. 


Experiment No. 563, subject P. K.: 

Salmon (in form of fish loaf) 


Grams. 

1,286.0 

.553. 

325.0 

717.0 

60.0 


Grams. 

796.4 

417.5 

22.4 

606.6 


Grams. 

320. 

13.8 

26.3 

2.9 


Grams. 

139.4 

0.6 

43.6 

3.6 


Grams. 


Grams. 

30.2 

5.5 

2.6 

2.1 


Potato 


115.6 
230.1 
101.8 
60.0 


Crackers 


Frixit 


Sugar 












Total food consumed 


2,941.0 
■49.0 


1,842.9 


363.0 

16.4 

346.6 


187.2 

6.2 

181.0 






Feces 




Amount utilized 




488.4 


33.1 








Per cent utilized 






95.5 


96.7 


96.2 


81.9 








Experiment No. 564, subject C. J. W.: 

Salmon (in form of fish loaf) 


1,115.0 
431.0 
177.0 
842.0 
105.0 


690.5 

325.4 

12.2 

712.3 


277.4 
10.8 
14.4 
3.4 


120.9 

0.4 

23.7 

4.2 




26 2 


Potato 


90.1 
125. 3 
119.6 
105 


4.3 
1.4 
2.5 


Crackers 


Fruit 


Sugar 














Total food consumed 


2,670.0 
56.0 


1, 740. 4 


306.0 

23.6 

282.4 


149.2 

7.4 
141.8 


440.0 

18.4 

421.6 




Feces 


6.6 
27 8 


Amount utilized 











Per cent utilized 






92.3 


95.0 


95.8 


80 8 










Average food consumed per subject per day 


745.4 


466.2 


98.0 


49.6 


121.0 


10.6 



Summary of digestion experiments with salmon in a simple mixed diet. 



Experiment No. 


Subject. 


Protein. 


Fat. 


Carbo- 
hydrates. 


Ash. 


561 


H. R. G 


Percent. 
91.9 
91.7 
95.5 
92.3 


Percent. 
96.9 
88.7 
96.7 
95.0 


Per cent. 
96.6 
94.0 
96.2 

95.8 


Percent. 
85.8 


562 


A. J.H.. .. 


76.9 


563 


P. K 


81 9 


564 


C. J. W 


80.8 




Average 






92.9 


94.3 


95.7 


81.4 









In the test periods in which canned salmon was included in the 
experimental ration, the subjects ate an average of 355 grams of 
salmon daily. The total diet eaten supplied 98 grams of protein, 50 
grams of fat, and 121 grams of carbohydrates per day, and the coef- 
ficients of digestibihty were found to be 92.9 per cent for protein, 
94.3 per cent for fat, and 95.7 per cent for carbohydrates. The fuel 
value of tlie diet was 1,326 calories. When allowance is made for 
tlie undigested protein and fat residues occurring in tiie feces result- 
ing from tlie accessory foods of the diet, the digestibility for the 
protein and fat of salmon is found to be 93.2 per cent and 93.7 per 
cent, respectively, and indicates that salmon is very completely uti- 
lized by the human body. 



14 



BULLETIN 649, U. S. DEPARTMENT OF AGEICULTURE. 



SUMMARY. 

In the study of the digestibility of the protein and fat suppHed 
by some common varieties, fish in the form of ''fish loaf" was served 
as the major part of a simple mixed diet, which also included pota- 
toes, crackers, fruit, sugar, and tea or coffee. The principal results 
are summarized in the following table: 

Summary of results of all digestion experiments. 



Number of 
experiments. 



Kind of fish. 



Average 
amount of 
fish eaten 
per man 
per day. 



Digestibility 
of fish 
protein. 



Digestibility 
offish fat. 



Grams. 



3... 
3... 

8... 
4... 



Mackerel . 
Butterflsh 
Crayfish . . 
Salmon . . . 



448 
471 
440 
355 



Per cent. 
93.1 
91.9 
92.8 
93.2 



Per cent. 

95.2 
86.4 
94.3 
93.7 



On an average, the subjects ate 448 grams of Boston mackerel, 
471 grams of butterfish, 440 grams of grayfish, and 355 grams of 
salmon daily, which would indicate that in every case the fish was 
eaten with relish. 

Considering the experiments as a whole, the total diet supplied on 
an average 99 grams of protein, 60 grams of fat, and 160 grams of 
carbohydrates daily, the fuel value being 1,576 calories. The low 
amount of fat and of carbohydrates was due to the fact that butter 
and similar fat were omitted and the foods other than fish loaf which 
supplied both protein and carbohydrates, were limited in order that 
both the fat and the protein in the diet might be contributed in as 
large proportion as possible by the fish. 

The average coefficients of digestibility for fish proteins were: 
Boston mackerel, 93.1 per cent; butterfish, 91.9 per cent; gra3rfish, 
92.8 per cent; and salmon, 93.2 per cent. In view of the close 
agreement, it would seem, from a dietetic standpoint, that the differ- 
ent fishes studied would supply protein in equally available form. 

The average coefficients of digestibility of the fish fats were found to 
be as follows: Boston mackerel, 95.2 per cent; butterfish, 86.4 per 
cent; grayfish, 94.3 per cent; and salmon, 93.7 per cent. As these 
figures show, the fats were well assimilated in the case of the mack- 
erel, grayfish, and salmon, which, according to the usual custom, are 
to be regarded as ''fat fishes." Considering the experiments as a 
whole, the very complete utilization of the protein and fat supphed 
by the fishes studied offer additional experimental evidence that 
fish is a very valuable food and that its extensive use in tiie dietary 
is especially desirable. 



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